1. Field of the Invention
The present invention is a method of position fixing, more particularly it is a method of position fixing an active transmitter source utilizing differential doppler.
2. Prior Art
Most prior art position fixing schemes can be categorized into three types of systems; the time-difference-of-arrival, hyperbolic system; the single-doppler-frequency system; and the phase-front-line-of-position system. The time-difference-of-arrival, hyperbolic system requires a distinguishing property in modulation, although stability of the signal emanating from the source to be located is not a requisite. The differential doppler technique of the present invention requires no modulation; however, this technique can operate on a signal with a very low band width, such as a single sinusoidal frequency, if desired. The differential-doppler technique of the present invention does not depend on modulation on the carrier, whereas the time-difference-of-arrival technique does.
Prior art single-doppler-frequency systems require high stability transmissions, although they do not require modulation. The differential doppler technique of the present invention does not require this stability as the frequency errors due to instability are minimized in applying the differential doppler technique of the present invention. The differential doppler technique of the present invention requires neither stability nor modulation, depending only on the change in frequency of the basic carrier, and not on the modulation on the carrier, in order to position fix an active transmitter source.
Differential doppler range techniques, such as disclosed in U.S. Pat. No. 3,397,398 issued to O. B. Dutton et al., utilize an active system-passive source in order to accomplish differential doppler range measurement. Two CW signals, having different frequencies, are transmitted in order to obtain two doppler frequencies which are phase different. This technique is a return measurement system utilizing cooperative transmitters, which does not have the utility of the technique of the present invention. The source in the present invention could be unstable, and the differential doppler range technique could not be utilized to locate such an unstable transmitter. The differential doppler range technique only requires a return for range, and does not utilize an external source of emanation in order to locate this source; rather the system locates a reflecting body, not a transmitter. Doppler range measuring systems utilize an active receiver, which transmits the signal it desires to receive and a passive reflecting body; rather than a passive receiver and an active source, as is in the present invention.
The differential doppler technique of the present invention offers the advantage of placing no restrictions on massive movement of the passive receivers, nor on antenna structural size or placement. The only requirement is that the received signals are detectable. A transmitter position fix may be calculated by data collected during a maximum of three contiguous measurement intervals, and these measurement intervals need not occur directly over the active transmitter source to be located, nor within close proximity to it, as long as the received signals are detectable.